Neuroblastoma (NB), a malignant neoplasm of neural crest origin, is the most common extracranial solid tumor in children. Spontaneous differentia- tion of NB to benign ganglioneuroma occurs occasionally in vivo. This process involves the loss of the neuroblast phenotype and the appearance of cells resembling mature ganglionic neurons. Even in NBs which do not mature completely to benign ganglioneuromas, the degree of histologic maturation directly influences the clinical behavior of the tumor and is, therefore, of clinical importance. Because most human neuroblastoma cell lines undergo morphologic and biochemical differentiation in response to a variety of gents including retinoic acid (RA), an excellent in vitro model for the examination of NB differentiation is available. The focus of this study will be to isolate and characterize the genes in NBs in which expression is either induced or down-regulated 180 minutes after treatment with RA by differentially screening a cDNA library constructed from LA-N-5 NB cells treated with RA for 180 minutes with radiolabeled cDNA probes prepared from untreated and RA-treated human NB cells (LA-N-5. By con- centrating on early changes in expression, genes which may be necessary for the implementation of the differentiation process rather than the genes whose products are associated with the specific neural features of the differentiated cell may be isolated. Other NB cell lines, many of which have been established an characterized in this laboratory, will subsequent- ly be probed with the differentiation related sequences to determine if these clones are expressed after RA treatment in other N-myc amplified and unamplified lines. In addition, the expression of these genes in primary tumors and the relationship between the levels of expression and the degree of histologic differentiation, and clinical outcome will be determined. Finally, these sequences will be incorporated into expression vectors and introduced back into human NB cells. Biological effects of the expressed clones in recipient cells will then be analyzed. Understanding the molecular mechanisms of NB differentiation may provide insight into the altered regulation of differentiation associated with NB.